Other victims of China's "black jails" are also thought to have been abducted and held captive by agents working for local politicians who fear that any investigation into alleged corruption could jeopardise their position and reputation.

The facilities, which allow local authorities to bypass the law, are hidden down backstreet's and alleyways. Some are nondescript "hotels", the owners of which can make up to $35 per prisoner, our correspondent discovered.

But Beijing denies any knowledge of the jails.

Jiang Yu, a spokesperson for the foreign ministry, responding to a question by Al Jazeera on the use of "black jails" in China, said: "I don't know what prompted you to ask this question.

"Things like this don't exist in China. China is a country with the rule of law, and everything is handled according to the law."

Rights groups, though, have questioned the extent to which Beijing is unaware of the secret prisons.

Petitioners punished

Wang Songlian, the research director of China Human Rights Defenders, told Al Jazeera that some facilities in Beijing functioned as centralised black jails and were used to house petitioners caught by Beijing police.

"Officials or staff in these centralised black jails would register the petitioners and then send them to the appropriate black jails organised by local authorities," she said.

The central government has deniedthat the 'black jails' exist

"So this is actually quite an organised and systematic system in which petitioners are sent from one black jail to another in order to punish them for bringing their complaints."

She said the central government used the petitioning system to give citizens hope, but had no interest in hearing their complaints or to address them.

"When people suffer injustice they can go to court to sue the local officials, but because of the dependence of the court on the local government it is not impartial," she said.

"The other option is for people to escalate their complaints [by petitioning] to the higher authority in the hope of getting some redress."

China's official media often carries reports about petitions being addressed by local governments.

"The government has an interest in keeping people's hope in the petitioning system alive," said Wang.

"Otherwise they will have all that pent-up anger in them and not have any outlet."